Field
The disclosure generally relates to hardfacing weld overlays used to protect components subject to abrasion
Description of the Related Art
Hardfacing and/or weld overlays are commonly used in a variety of applications to protect components from excessive material loss in abrasive environments. In certain applications it can be desirable for the hardfacing overlay to remain crack free after the welding process is complete. This is not typical of hardfacing alloys as many stress relieve crack during or immediately after welding due to the inherent low toughness of the alloy. Hardfacing alloys are typically very hard, in excess of 50 HRC, and it is well known by those skilled in the art of metallurgy that hardness is generally inversely related to toughness. In certain applications, it is further desirable to weld over the original worn down layer of the hardfacing, and to do so without generating cracks in the original or secondary weld overlay. In such applications it is often desirable to continuously weld over worn layers again and again as wear occurs on the existing hardfacing to continuously repair and re-build the hardfacing layer. In the process of re-building hardfacing layers in this manner, it is commonplace to weld a hardfacing alloy of one type over a hardfacing layer of a second different type. It is generally desirable to do be able to do so without introducing any loss in performance.
Two types of cracks are known to commonly occur in hardfacing. The first, stress cracks, are very common and easy to detect. These cracks occur due to the weld bead contracting as it cools, resulting in thermal stresses being built up in the weld to a level which creates cracks in the weld. The second, hot tears, are less common, and less detectable as they do not create loud crack ‘ping’s during the welding process. Hot tears occur when the alloy solidifies over a larger temperature, and the edges of the bead begin to solidify and contract while the center of the bead has not yet fully solidified. The contraction of the outer bead pulls the liquid phase at the center of the bead apart. This mechanism is also known to those skilled in the art of metallurgy.
An example of a specific application where the problems of stress cracking and hot tearing commonly occur is hardbanding. Hardbanding is the process of protecting the drill pipe, generally in the form of two or more weld beads deposited onto the tool joint. It is common and also desirable to deposit additional overlays onto worn hardbands to rebuild the wear resistant surface. As drill pipe is commonly rented out to various drilling sites, and it is also common to apply one type of hardbanding alloy over a second hardbanding alloy of a different type. The current available selection of hardbanding alloys is relatively diverse; many form wear resistance through the carbide formation, and many others through boride formation. Welding a carbide forming alloy over a boride forming alloy creates conditions where the re-building layer is very likely to either stress crack or hot tear.
One example of an alloy which is highly resistant to hot tearing, but is subject to stress cracking is presented in U.S. Pat. No. 8,647,449: Fe: bal, Cr: 5, Nb: 4, V: 0.5, C: 0.8, B: 0.9, Mo: 3.5, Ti: 0.2, Si: 0.5, Mn: 1, hereby incorporated by reference in its entirety. This alloy can be welded as a single layer, but is increasingly likely to stress crack with subsequent re-building layers. The general class of materials which exhibit this type of behavior can form either primary carbides or borides, and also form eutectic carbides or borides in excess of 15 volume %. This classification is based upon extensive research conducted within this study. An example of such an alloy is shown in FIG. 1 (left), where the both carbides [101] and borides [102] exist in a ferritic matrix [103], but the eutectic borides are present at about 20 volume %.
One example of an alloy which is highly resistant to stress cracking, but subject to hot tearing is that presented in WO 2014/127062 (which claims priority to U.S. 61/889,548, filed Nov. 4, 2013): Fe: bal, C: 1, Cr: 5, Mn: 1.1, Mo: 0.75, Ni: 0.1, Si: 0.77, Ti: 3; WO 2014/127062 and U.S. 61/889,548 are hereby incorporated by reference in its entirety. This alloy can be re-built over worn layers of a similar type indefinitely without stress cracking or hot tearing. However, when this alloy is welded over a worn hardbanding layer containing B, it will hot tear. The general class of materials which exhibit this type of behavior form either primary carbides or borides (carbides or borides which precipitate from the liquid prior to the austenite matrix phase), but do not form both carbide and boride. An example of an alloy susceptible to hot tearing is shown in FIG. 1 (right), where primary carbides [104] are embedded in a martensitic matrix [105]; the lack of any eutectic carbides or borides, and the lack of borides in the microstructure create increased likelihood for hot tearing.
The current state of the art for hardfacing materials possess alloys which fall into either of these two general categories. Thus, there is a need for a class of hardfacing materials which are resistant to both of these forms of failure: stress cracking and hot tearing.